Depends on the type of rivets
There are a couple of different alloys used to make aircraft rivets, and they have different part numbers.
Our normal AN-470-AD and AN-426-AD rivets are made from 2117 aluminum and are already age-hardened to a certain extent. They should not normally get harder with more age, so they don't have a shelf-life.
There are some rivets, often called "refrigerator rivets" that are in a solution-treated state (soft) and will age-harden over time at room temperature, so they are stored cold. These are the type of rivets that will get too hard to drive over time if not stored correctly, and the shop-formed head may crack when driven.
There is a procedure for re-solution-treating these rivets to rejuvenate them, making them softer so you can drive them.
I think that you would typically find these rivets in larger sizes, and in the aerospace industry where higher strength would be called for, so it is plausible that you would find some of these in a Boeing supply stock - although if they were not being stored cold, they probably were surplus.
To AARVIG, who asked if old rivets crack, does that mean they would go bad on the airplane, the answer is no, just the opposite! These rivets are getting stronger over time, not weaker. But by getting harder, they are less formable, and so the tails may crack, or the heads crack if yout try to buck them. But a soft rivet, bucked, then age-hardened, gets stronger and better.
Age-hardening aluminum, normally done by heat treatments, but most age-hardening alloys continue to 'age' and gain strength for a period of time even at room temperature. This is cool. For big airplanes that have thick, complex-shaped wing skins, you get a fresh slab of aluminum, machine it and form it into your wing skins (usually rolling, sometimes even shot-peening to form compound curves), drill the holes, put the wing together, and then, as it moves its way up the assembly line, over weeks and weeks, the wing skins are getting stronger as the aluminum age-hardens.